1. Field of the Invention
The present invention relates generally to passive infrared (PIR) motion sensors.
2. Related Art
Passive infrared motion sensors generally consist of several features. An optical element (such as a lens or mirror) and an infrared (IR) detector together define and collect radiation from a field-of-view (intersecting and thus defining a monitored spatial volume), from which the optical element conveys radiation onto an infrared (IR) detector which is generally responsive to mid-IR light in the 6-14 micron wavelength range. The detector, in turn, provides an electrical signal responsive to changes in the effective blackbody temperature of the surfaces of objects within the monitored volume and radiating toward the optical element, which signal is passed to analog processing circuits, which, in turn, create a digital signal that may be directly or indirectly compared to a certain threshold amount of temperature change “seen” by the optical element from within the monitored volume. The digital signal may be further processed by logic circuits in order to provide a desired output indication, for example, of a warmer human crossing in front of cooler objects or background within a monitored volume. Upon detection of changes in radiation as a result of temperature difference between a moving “target” (e.g. a human) and its background, motion sensors generally transmit an indication to a host system, which may in turn activate an intrusion “alarm”, change room lighting, open a door, or perform some other function.
An infrared motion sensing system typically comprises an optical-element (lens/mirror) array disposed to direct IR radiation from humans to a juxtaposed IR detector from within volumes to be monitored. Other radiation (e.g. visible and near-infrared [NIR] light) passing through the array to the IR detector is superfluous and can cause false motion sensing, because high levels of such radiation can cause the detector to emit signals even in the absence of IR radiation from within the monitored volumes. To prevent such false motion sensing, IR detector elements are typically placed in opposed-polarity pairs, so that superfluous radiation (not focused by the sensor's IR optical-element array) falls about equally on both elements, which produce approximately equal and opposite signals that cancel each other. However, this practice means that the sensor's optically-monitored volumes are also in opposed-polarity pairs, which places limits on the monitored-volume design.
PIR motion sensors typically employ pyroelectric IR detectors to measure changes in IR radiation intensity. FIGS. 1A to 1C show two views and a schematic symbol of a simple prior art pyroelectric detector 10. Such detectors operate by the “piezoelectric effect”, which causes electrical charge migration in the presence of mechanical strain. Pyroelectric detectors take the form of a capacitor—two electrically conductive plates 12 separated by a dielectric 14. The dielectric is often a piezoelectric ceramic. When IR radiation causes a temperature change (and thus some mechanical strain) in the ceramic, electrical charge migrates from one plate to the other. If no external circuit is connected to the detector, then a voltage appears as the “capacitor” charges. If an external circuit is connected between the plates, then a current flows, depending on the resistance of the circuit
In either the voltage or current case, a pyroelectric detector's “signal” is very small. These detectors are sensitive, delicate and subject to error due to minute, unintended electrical currents. Therefore, they are typically produced in “clean room” environments and housed in dry-nitrogen-filled hermetically sealed cases. This and other factors cause the pyroelectric detector to be a significant part of the cost (5-10%) of a typical PIR motion sensor, so most PIR motion sensors employ only one or two such detectors.
In order to monitor a large space with only one or two detectors, a typical PIR motion sensor is designed with an array of multiple optical elements (e.g. lenses or mirrors) on the surface of the sensor. FIG. 2 shows a simple prior art PIR motion sensor 15 which has an array of optical elements 16 monitoring a space divided into sub-volumes or monitored volumes 17 corresponding to the fields of view defined by each optical element and the detector. Each optical element focuses the IR radiation from objects within a sub-volume of the monitored space into an image appearing over the detector 10. Since the optical elements can be manufactured inexpensively, the optical array may comprise up to fifty or even more optical elements, with each optical element directing radiation to the detector from a separate sub-volume 17 of the monitored space.
The sub-volumes can be interleaved with non-monitored sub-volumes, so that a radiation-producing target (e.g. a human) passing from sub-volume to sub-volume causes a “target radiation/background radiation/target radiation” pattern at the detector. In the case of humans, since some parts of humans are nearly always radiating with different intensity than that of the background (due to different temperature from that of the background), this pattern causes changing radiation (and thus changing temperature) of the detector. The resulting piezoelectric currents and/or voltages are wave signals versus time, which can be sent through a band pass filter, amplified and sent to the sensor's signal processor for determination of motion by evaluation of the number of wave peaks that exceed a designated reference level.
Over the two decades that pyroelectric detectors have formed the basis for PIR motion sensors, many improvements have reduced the probability of false alarms. To prevent visible and near-infrared light from reaching the detector, optical filters have been added as detector windows. Also, coatings (in the case of mirrors) and additives (for lenses) have been added to prevent the sensor's optical element array from focusing of visible and NIR light onto detectors. These steps have reduced the possibility of PIR motion sensors producing false alarms due to signals caused by, for example, automobile headlights shining through building windows. However, there are practical and cost limitations to such improvements, so it is desirable to employ methods by which the detector itself can emit smaller signals in the presence of superfluous radiation.
One particular method for prevention of false alarms due to interfering visible and NIR light is to place detector elements in pairs of equal size and opposite detection polarity, so that light not focused by sensor's optical element array (i.e. non-IR radiation) tends to be equally incident on both elements, thus causing the signals from the equal and opposite elements to roughly cancel one another. FIGS. 3A to 3D shows three views and a schematic symbol of a typical prior art dual-element detector 20 which has detector elements 22, 24 of equal size and opposite direction polarity. Cancellation is not perfect, as the elements are not exactly equal, and the radiation not exactly uniform. However, the effect improves rejection of non-focused radiation (over that of single-element detectors). Equal and opposed elements can also reduce the undesirable signals resulting from other types of non-intrusion stimuli, such as shock and temperature change.
Detector elements are often of an aspect ratio between 1:2 and 1:4, to approximate the distribution of far infrared radiation from an upright human. FIG. 4A shows the pattern of monitored sub-volumes 25 in a prior art dual-element sensor system 26 resulting from mounting dual opposed-polarity detector elements as in FIG. 3E (aspect ratio 1:2) behind the same optical-element array 16 as in FIG. 2. FIG. 3E is a “functional diagram” illustrating the aspect ratios and juxtaposition of the longitudinal cross-section of monitored sub-volumes 25 arising from the detector elements. As in a sensor with a single element detector, the sub-volumes can be interleaved with non-monitored sub-volumes, so that a radiation producing target (e.g. a human) passing from sub-volume to sub-volume causes a “target radiation/background radiation/target radiation” pattern at the detector. In this case, wave peaks of alternating polarity are generated due to the opposed-polarity elements, as illustrated in FIG. 4B. Again, as in the prior art sensor of FIG. 2, the resulting piezoelectric currents and/or voltages are wave signals versus time, which can be sent through a band pass filter, amplified and sent to the sensor's signal processor for determination of motion by evaluation of the number and polarity of the wave peaks that exceed a designated reference level.
There are some problems with the prior art dual element detector system of FIG. 4A. Because the motion sensor's detectors are generally placed at the focal point of the optical elements, the monitored sub-volumes are a projection of the detector elements' shapes, ever-expanding at an angle defined by the optical focal length and the detector element size. This can be a problem in designing a special kind of motion sensor commonly called a “curtain” sensor. This type of sensor is designed to monitor a long, narrow volume (as might in an actual application be a long, narrow rectangular prism-shaped volume between museum visitors and the objects that they are viewing). Seen from the side, a curtain sensor's set of monitored volumes (being defined by juxtaposition of several optical elements and one dual-opposed-element detector) usually covers a near-90° vertical angle, as may be seen in FIG. 5, in which five pairs of sub-volumes 25 (each similar to the pair shown in FIG. 4A) are defined by a dual-opposed-element detector and five lenses. The so-called “curtain” is a set of monitored volumes which occupies a rectangular prism more high and wide than it is deep, and which is shaped somewhat like the volume occupied by a pleated curtain.
FIG. 6 is a top view of the monitored volumes 25 of the prior art system of FIGS. 4A and 5. Due to the angle, the monitored volumes' shape is not really that of a “curtain”, in that the volume grows wider with distance from the sensor. The curtain's shape may be made more constant by using optical elements of different focal lengths for each vertical angle. This type of system is commonly achieved with mirrors of varying focal length. FIG. 7 is a top plan view illustrating a prior art mirror design curtain sensor 38 having monitored volumes 30A, 30B; 32A, 32B; 34A,34B; 35A,35B, and 36A,36B, where the mirror elements are of different focal lengths so that the angular width of each field of view or monitored volume is different from the other fields of view or monitored volumes on each side of the sensor, while the angle between each pair of fields of view (i.e. 30A and 30B, 32A and 32B, and so on) is approximately equal. The focal lengths are arranged so that the angular width is smaller for the longer fields of view, with the longest field of view or monitored volumes 30A and 30B having the smallest angular widths, thus allowing the “curtain” 40 to remain narrow at longer distances from the sensor, unlike the prior art lens design of FIG. 6. However, sensor systems using mirrors are more expensive and more difficult to manufacture than lens-based systems. Curtain sensor systems with more uniform curtain shape are also possible with lenses of varying focal length, yet, in common motion sensors where lens arrays form the sensor's front face, the design causes a problem of undesirable sensor appearance, since the lenses must be positioned at varying distances from the (shared) detector, producing an unsightly and uneven appearance to the front face of the sensor.